Hey Guys - First off I want to say thanks for all the great information. Been lurking for a while. Anyways I'll get to the point - I know there are a lot of threads on this subject but I am hoping someone can help me with my specific problem.

Car: 2008 Subaru WRX Millage: 81,XXX

Basically I have been dealing with air entering the Cooling System and cannot figure out how to rectify the situation. The symptoms are indicative of a cooling system air bubble - temp gauge will shoot up, then return to normal. Heater will stop working for a short period then work again.

If I burp the system or even keep it topped off the temp doesn't fluctuate. I can go about 75-100 miles before the bubble starts to effect the system.

Interestingly if I barely fill the plastic overflow, when the car is driven (10-20 miles) it is either overflowing or right at the top. Once the car some fluid goes back into the system but the overflow is left with more than it started with. This continues until fluid flows out of the overflow into the engine compartment.

Background: About 1000 miles ago the turbo on the car went out. It was replaced with a re-manufactured one. After less than 500 miles on the new turbo I went to pass someone and the turbo made a very loud noise, then the waste gate dumped. It tripped an engine code. When I parked the code was determined to be "Turbo Over-boost". The cooling system problem started at exactly this same time.

I brought it back to the mechanic who did the turbo and he said the over-boost was due to a vacuum line not being reconnected and that the over-heading problem was completely unrelated and the timing was a coincidence.

He diagnosed as a head gasket and wanted $3,000 to fix. I took to another mechanic, not feeling good about this "coincidence" and he did a head gasket chem test on it twice. Both times it came back clean. Pressure test of the cooling system was good as well. He replaced both radiator caps thinking that was the issue but the problem is still there. There is no exhaust gas in the cooling system, but air is still getting in.

Is it possible that the turbo has a crack (or bad seal) and that clean, pressurized, intake side air is being forced from the turbo into the cooling system??? I did not small bubbles in the overflow (near turbo) tank that are almost nil at idle and increase with RPMs. If not, does anyone have any other good ideas?